This is a small change, but it still might be difficult to review.

Related:

• Make DragGestureRecognizer abstract methods public #151446
• Proposal: use "enhanced enums" in the repo for reduced boilerplate #147799

Motivation

Dependency conflict

Currently, package:flutter/painting.dart depends on package:flutter/gestures.dart, so anything within the gestures library cannot use something declared in painting (since that would create a dependency loop).

This resulted in a less-than-ideal situation: the DragGestureRecognizer needs to use an Axis value for its primaryDragAxis field and its getSumDelta() method, but it isn't allowed to import the Axis enum.

So instead, the enum was copied into the file and given a private name. All methods that use this enum have to be private as well: if someone needs a version of PanGestureRecognizer with a slight change to the distance required to accept the gesture, they need to copy the entire DragGestureRecognizer interface and re-implement each abstract method before tweaking to the desired behavior.

Enum enhancements

Axis and AxisDirection were given a few helper methods before enhanced enum features were added to Dart. This provides an opportunity for a satisfying refactor:

// before
if (axisDirectionToAxis(intent.direction) == axisDirectionToAxis(state.direction)) {
  // ...
}


// after
if (intent.direction.axis == state.direction.axis) {
  // ...
}

// before
Flex(
  direction: flipAxis(axisDirectionToAxis(direction)),
  // ...
);


// after
Flex(
  direction: direction.axis.flipped,
  // ...
);

It's much easier for me to understand the process of "convert the AxisDirection to an Axis, and then flip it" when it isn't a nested function call that's read from right-to-left.

Summary of changes

Migration

Since painting/basic_types.dart is already exporting a type from the foundation package, the migration was more-or-less trivial: enums in that file were moved into foundation/basic_types.dart.

Deprecation

Axis and AxisDirection were given a .flipped getter that corresponds to flipAxis() and flipAxisDirection() respectively.

AxisDirection also has .axis and .isReversed that corresponds to axisDirectionToAxis() & axisDirectionIsReversed().

I also added @Deprecated() to the functions being replaced; unfortunately at this point we can't implement a data-driven fix for a "function → getter" migration. Perhaps instead of deprecating now, we could do something similar to WidgetStateProperty.all: